1. Field of the Invention
The invention relates to a control circuit layout for technological processes having large control ranges.
2. Description of Related Art
Conventional control circuits for technological process installations typically include a control element inserted into the process or control system. The setting of the control element is regulated by means of the control circuit so that a controlled variable is maintained at a desired set value. In the case of processes in which the set value of the controlled variable may vary within a wide range as a function of demand, the control element must be designed so that a large range of set values of the controlled variable is possible. The regulating range of the control element must then be chosen accordingly. This, however, necessarily reduces the regulating efficiency of the control element in the area of small set values of the controlled variable, i.e., in the lower load range of the process. The accuracy of control in this range (light load regulation) must be effected, for example, at least during the entry into and exit from the normal load, and therefore regulation is frequently inadequate.
A conventional, single-loop control circuit layout with only one control element inserted in the control system is no longer satisfactory for the exact regulation of many types of technological processes.
DE 28 39 476 B2 discloses a control apparatus with two two-position controllers connected in parallel, wherein controllers have different switching hysteresises and are set for the same set value of the controlled variable. These controllers regulate two control elements located in a single control system. Depending on the load demand, neither of the control elements, or one, or both, are actuated. At the switching points, merely the simple deactivation or actuation of one control element is provided without coordination with the other control element, so that the transitions are not without jolts, thereby causing breaks to appear in the actual value curve of the control element.
In an electronic control apparatus disclosed in DE 25 10 992 B2, a single regulator actuates, with a two-position and a three-position outlet, four similar control elements located in a single loop control system. The control elements are deactivated or actuated as a function of the regulation difference and hysteresis of the controller. In case of higher load demands, here again several control elements are actuated simultaneously. Under this control, the actuation and deactivation of control elements are again not without jolts.
DE 35 17 008 C3 discloses a circuit layout with a two-stage servo amplifier and a single control element. The two stages of the servo amplifier alternate processing the input signal for the common control element whenever a limit value of the controlled-variable is exceeded or not attained. The switching between the electronic servo amplifier stages is effected by means of voltage threshold switches. The provision of only a single control element leads to the fact that in the light load range regulations can be carried out with a lesser sensitivity than in the normal load range.